The substantial increase in demand for semiconductor light emitting devices, and the corresponding increase in competition to satisfy demand has caused manufacturers to seek techniques that will reduce costs or improve performance. Of particular note, techniques that improve the efficiency or quality of the emitted light may serve to distinguish one competitor's product from the others.
FIG. 1 illustrates an example prior art Thin Film Flip Chip (TFFC) InGaN Light Emitting Device (LED), such as disclosed in U.S. Pat. No. 6,828,596, “CONTACTING SCHEME FOR LARGE AND SMALL AREA SEMICONDUCTOR LIGHT EMITTING FL1p-CHIP DEVICES”, issued to Daniel A. Steigerwald, Jerome C. Bhat, and Michael J. Ludowise, and incorporated by reference herein.
In this example device, a light emitting layer 120 is formed between an n-layer 110 and a p-layer 130. An external power source (not illustrated) provides power to the device via connections to pads 160 and 170. The p-pad 160 is coupled to the p-layer 130 via a p-contact 140, through an optional guard layer 150 that inhibits migration of the p-contact material. The n-contact layer 170 is coupled directly to the n-layer 110 in this example. A boundary layer 180 isolates the n-contact layer 170 and n-layer 110 from the p-layer 130 and p-contact 140.
The p-contact 140 is provided over a large area to facilitate a uniform distribution of current through the p-layer 130, which has a relatively higher resistance to current flow. The n-layer 110 does not exhibit a high resistance, and thus the n-contact covers a smaller area, which may be 10% or less of the device area. The p-contact 140 is preferably highly reflective to reflect the light toward the top, emitting surface of the light emitting device. Silver is commonly used as the p-contact 140. The n-contact layer is also reflective and metals such as Aluminum are preferred. The guard layer 150 may be metallic, but is only partially reflective as a suitable highly reflective metal has not yet been found for this application. This partially reflective guard sheet fills the area adjacent to the p-contact, resulting in higher optical loss at the p-contact periphery.
The inventors have recognized that the light generated within about 15 microns of the periphery of the p-contact may, with high probability, enter the guard layer area 150 and suffer optical absorption before having a chance to exit the device. Therefore, current injected at the edge of the p-contact will exhibit a lower external quantum efficiency than current injected at the center area of the p-contact.
Despite the greater optical loss of the edges and corners of the device, the inventors have also noticed that more emitted light is produced at the periphery and in the corners than at the center of the device, because the voltage drop associated with the lateral flow of current through the n-contact layer, combined with the exponential dependence of vertical current flow upon junction voltage, provides a significantly higher current density at the edges and in the corners of the device. These relatively high injection currents create a slight halo-effect, with bright areas in the corners of the device.
In addition to potentially introducing optical anomolies, such a non-uniform current injection pattern is inefficient, as the internal quantum efficiency is lower for higher current densities. The ‘over-emitting’ portions, particularly the corners, of the light emitting device will also be ‘hot-spots’ that draw more current in the device, which have been observed to lead to premature failure of devices operated at high current.